Materials technology pursues the development of new and useful commercial materials including new hard materials. Such new hard materials include without limitation sintered ultra-fine powdered metals, metal matrix composites, heat treated steels (hardnesses of between about 50 to 60 Rockwell C), and high temperature alloys. These new materials have been developed to have extraordinary combinations of properties, such as, strength, toughness, stiffness or rigidity, hardness, and wear resistance, that makes them very suitable for uses in heavy industries, aerospace, transportation, and consumer products.
These extraordinary combinations of properties present challenges to the application of existing manufacturing and finishing processes to the new hard materials. Quite simply, these materials are very difficult and expensive to drill, cut, and form. For these new hard materials to realize the full extent of their commercial potential these challenges must be overcome. One can best address these challenges by the use of strong cutting tools that use a superhard material.
Superhard materials are significantly harder than any other compound and can be used to drill, cut, or form other materials. Such materials include diamond and cubic boron nitride (cBN). Diamond has a Knoop 100 hardness from about 75-100 gigapascal (GPa) and greater while cBN has a Knoop 100 hardness of about 45 GPa. Boron carbide (B.sub.4 C) and titanium diboride (TiB.sub.2), the next hardest materials, each have a hardness of only about 30 GPa.
Diamond is found in nature and can be synthesized. Boron nitride, including cBN, is synthetic (see e.g., U.S. Pat. No. 2,947,617, in the name of Wentorf Jr.). Both synthetic diamond and synthetic cBN are produced and then sintered using high-temperature high-pressure (HT-HP) conditions (about 5 GPa and about 1500.degree. C., see e.g., Y. Sheng & L. Ho-yi, "HIGH-PRESSURE SINTERING OF CUBIC BORON NITRIDE," P/M '78-SEMP 5, European Symposium on Powder Metallurgy, Stockholm, Sweden, June 1978, pp. 201-211.).
Presently, the two primary superhard commercial cutting tools comprise a polycrystalline diamond (PCD) cutting tool and a polycrystalline cubic boron nitride (PCBN) cutting tool. The PCD cutting tools have their typical application in the machining of hard non-ferrous alloys and difficult-to-cut composites. The PCBN cutting tools typically find application in the machining of hard ferrous materials. In the typical polycrystalline (PCD or PCBN) cutting tool, the cutting edge comprises a HT-HP superhard tip brazed onto a carbide blank. The tip comprises micrometer sized HT-HP diamond or HT-HP cubic boron nitride (cBN) crystals intergrown with a suitable binder and bonded onto a cemented carbide support. The HP-HT manufacturing process, as well as the finishing process for these tips, each entails high costs. The result is that PCD cutting tools and PCBN cutting tools are very expensive.
In addition to the expense, these cutting tools usually comprise a single tipped tool wherein the tip has relatively few styles with a planar geometry. Even though these cutting tools are expensive and come in relatively few styles, presently they are the best (and sometimes the only) cutting tool suitable to economically machine new hard difficult-to-cut materials.
Through the development of techniques for the low pressure deposition of diamond one is able to deposit conforming layers (or films) of diamond on cutting tool substrates without any significant limitation to the geometry of the cutting tool. While the diamond-coated cutting tools have advantages over the PCD cutting tools, there remain some significant limitations to the use of diamond coated cutting tools.
One primary limitation with diamond cutting tools (i.e., PCD and coated tools) is that diamond oxidizes into carbon dioxide and carbon monoxide during high temperature uses. Another principal limitation with diamond cutting tools is the high chemical reactivity of diamond (i.e., carbon) with certain materials. More specifically, materials that contain any one or more of iron, cobalt, or nickel dissolve the carbon atoms in diamond. These limitations reveal that while diamond-coated cutting tools provide certain advantages, there is a universe of materials that require a cutting tool with a superhard coating, but for which the use of a diamond-coated cutting tool is inappropriate.
It is very apparent that there is a need to provide a cutting tool with an adherent superhard coating that overcomes the above extant problems with diamond-coated cutting tools. More specifically, there is a need to provide a cutting tool with an adherent superhard coating wherein the coating does not oxidize during high temperature use. There is also a need to provide a cutting tool with an adherent superhard coating wherein the coating does not chemically react with workpiece materials that contain any one or more of iron, cobalt, or nickel.
One superhard material that passivates through the formation of protective oxides (i.e., boron oxide(s)) and therefore does not oxidize at high temperatures is boron nitride. In addition, boron nitride does not chemically react with any one or more of iron, nickel, or cobalt so that a workpiece which contains any one or more of these components does not dissolve the boron nitride. These advantageous properties of boron nitride exist with respect to various crystalline forms thereof such as, for example, amorphous boron nitride (aBN), cubic boron nitride (cBN), hexagonal boron nitride (hBN), and wurtzitic boron nitride (wBN), wherein cBN has especially good properties.
Although it is technically feasible to synthesize boron nitride, including cBN, from gaseous precursors, adhesion to a substrate continues to present technical challenges. For example, some cBN coatings fragment shortly after deposition (see e.g., W. Gissler, "PREPARATION AND CHARACTERIZATION OF CUBIC BORON NITRIDE AND METAL BORON NITRIDE FILMS," Surface and Interface Analysis, Vol. 22, 1994, pp. 139-148.) while others peel from the substrate upon exposure to air (see e.g., S. P. S. Arya & A. D'amico, "PREPARATION, PROPERTIES AND APPLICATIONS OF BORON NITRIDE FILMS," Thin Solid Films, Vol. 157, 1988, pp. 267-282.). Thermal expansion mismatch between the cBN coating and the substrate creates extreme residual stresses and might explain fragmentation. The formation a weak layer between the cBN coating and the substrate by the reaction of hygroscopic compounds with ambient moisture might explain peeling.
For the foregoing reasons, there is a need for a coating scheme comprising a boron and nitrogen containing coating, preferably one comprising boron nitride and more preferably one comprising cBN, that satisfactorily adheres to a substrate. Preferably, the coating scheme should be applicable to a substrate to form tooling, such as chip form machining inserts, for drilling, cutting, and/or forming the new hard difficult to cut materials. Thus a method for making an adherent boron and nitrogen containing coating, preferably one comprising boron nitride and more preferably one comprising cBN, is needed.